Houston, Texas (CNN) -- John Phillip Hernandez and a friend walked into the Collector's Firearms gun store in Houston, Texas, to buy a cache of weapons. Hernandez was wearing sunglasses and a dark T-shirt with the words "I Am the Scene" scrawled across the front.

It was April 28, 2007. Collector's Firearms was one of two gun stores they visited that day as part of a scheme to arm Mexican cartels across the border, according to federal court documents.

Hernandez's friend passed his background checks and did all the buying, prosecutors say. On that spring day, he purchased six weapons, including a quick-firing 7.62-caliber firearm and a favorite cartel weapon, the Bushmaster .223.

Within days, ATF investigators say, those weapons were put on the road and funneled into the hands of cartel members.

"The cartels are looking to supply their private armies, and they are coming up with more elaborate, larger schemes," said Dewey Webb, the special agent in charge of Houston's Bureau of Alcohol, Tobacco, Firearms and Explosives field office.

The Houston case offers a glimpse into the underworld of illegal gun trafficking and how Mexican drug cartels find much of their firepower in the United States. Straw purchases are made by people who are legally qualified to buy firearms, but those weapons then make their way into the hands of criminals, authorities said.

The daily struggle plays out as drugs flow north and guns and money flow south. At least 55 murders were the result of the gun scheme that began in Houston gun shops, authorities said.

Hernandez is at the center of the case, what Houston ATF agents say is one of the largest straw-purchasing schemes they have ever seen.

"He doesn't stand out in a crowd," an ATF agent told CNN. "He's just a regular guy." The agent asked not to be identified because he's been intricately involved in the investigation of this case.

Federal investigators say Hernandez recruited and organized 23 people around Houston. Together the men funneled nearly 340 firearms valued at almost $370,000 to Mexican drug cartels. ATF agents say almost 100 of those weapons have turned up at cartel-related crime scenes in Mexico and Guatemala.

Hernandez pleaded guilty in 2009 to making false statements about firearms purchases. He's currently serving an eight-year prison sentence.

Of the 23 other people connected to the case, 11 have pleaded guilty for their roles in the gun trafficking scheme. Each was sentenced to less than eight years in prison. Several others struck plea deals and continue to cooperate with federal authorities.

What made this group so effective was its ability to appear like ordinary gun buyers. All the men had clean criminal backgrounds, authorities say. It wasn't until the firearms started showing up at violent crime scenes in Mexico that federal investigators started piecing together the magnitude of the case.

ATF investigators, cooperating with Mexican authorities, traced the weapons back to various Houston gun shops and started seeing the same names of repeat customers.

"It really surprised us at first that we had this many people linked together," said the ATF agent who spoke on condition of anonymity.

Federal investigators say Hernandez and his men were reimbursed by the cartels for the money spent on the weapons, and then paid an extra $100 to $200 for each firearm they supplied.

The weapons Hernandez purchased himself, according to federal court documents, were used in the kidnapping and murder of a prominent Mexican businessman. Another group of weapons turned up in a shooting known as the "Acapulco Massacre" in 2007. Seven people were slaughtered that day, including four police officers.

ATF agents say breaking up these networks of straw purchasers in the United States is a key battlefront in the fight against Mexican drug cartels.

"These folks that are out buying these guns, they're just as responsible as the people pulling the trigger and killing people in Mexico," said Webb, Houston's top ATF agent.